1. Field Of The Invention
The present invention relates to surgical prosthetic devices and more particularly to an improved orthopedic knee implant that includes a polymeric femoral component, preferably of an ultra high molecular weight polymeric material that articulates against metallic (or ceramic or ceramic coated metal) patella and tibia components.
2. General Background
About ten years ago, total knee arthroplasty became a popular and routinely accepted treatment for arthritic and other diseases of the knee. During this period of time and before, various combinations of articulating materials were tried with varying degrees of success. Polyacetyl type of polymeric material has been used for the femoral surface and articulating against another polymeric material (e.e. teflon) on the tibial surface. Polymer-polymer articulation has been tested but produced excessive wear. The most widely accepted combination used at present is a metal femoral surface articulating against polymer tibial and patella surfaces.
As the nature of the complex knee joint articulation became better understood, the system most widely used today, a cobalt alloy femoral surface and an Ultra High Molecular Weight Polyethylene (UHMWPE) tibial and patella surface became the standard. However, bioengineers, surgeons, and other scientists are still learning more about the performance of this knee system as sufficient numbers of patients are just now reaching more than ten years postop. In the past five years or so the medical community has come to appreciate the adverse effect of UHMWPE wear debris and its ability in sufficient volume to produce bone lysis and thus revision of the implant.
The most prevalent source of UHMWPE wear debris results from relatively rapid (compared to the tibial surface) wear of the UHMWPE patella surface. If the patella has a metal backing, this backing can eventually wear against the metal femoral surface following excessive polyethylene wear, and further aggravate the wear process and accelerate the adverse consequences via production of metal and UHMWPE debris.
Tibial wear of the UHMWPE has also been observed. Thinner UHMWPE surfaces on the tibia may allow eventual wear through to the underlying metal. Thicker tibial UHMWPE surfaces are desired but at the expense of excessive resection of the bone (tibia) during surgery. In just the past few years, the benefit of ceramic-UHMWPE wear combinations in the total hip joint and in laboratory tests have shown this wear combination to reduce friction and UHMWPE wear. Knee simulator tests in Japan have shown this to also be the case for monolithic ceramic knee femoral components articulating against UHMWPE.
Although the ceramic femoral surfaces show a clear advantage over cobalt alloy femoral surfaces in reducing UHMWPE wear, the fact remains that both the UHMWPE tibia and particularly the UHMWPE patella component (roughly twice the contact stress of the tibial surface) still wear at a finite rate. Alternative attempts to minimize UHMWPE tibial wear include the use of movable tibial segments such as in meniscal bearing total knee designs.
In all the currently available total knee systems both tibial and patella wear of the UHMWPE material occurs against the polished metal femoral surface. During wear of the knee, it is the UHMWPE which undergoes wear in a constant area, such as the domed patella surface, or in a relatively constant area such as in the tibial surface. That is, the wear of the UHMWPE is not minimized because a particular region(s) of the UHMWPE remains in contact with the mating metal surface during relative motion (sliding distance) of the metal surface; The wear factor K, of UHMWPE articulation against polished metal implant surfaces is the volume (mm.sup.3) of UHMPE per unit stress and sliding distance. Thus, for a given load (contact stress), reducing the sliding distance over which the metal surface rubs the UHMWPE will reduce the volume of UHMWPE wear debris. The use of both a polymer femoral and tibial and patella component does not eliminate this wear difference and, depending on the polymer combination, may actually increase wear dramatically.